Method of fabricating a semiconductor device with a wet oxidation with steam process

ABSTRACT

A method of fabricating a semiconductor device includes depositing a dielectric film and subjecting the dielectric film to a wet oxidation in a rapid thermal process chamber. The technique can be used, for example, in the formation of various elements in an integrated circuit, including gate dielectric films as well as capacitive elements. The tight temperature control provided by the RTP process allows the wet oxidation to be performed quickly so that the oxidizing species does not diffuse significantly through the dielectric film and diffuse into an underlying layer. In the case of capacitive elements, the technique also can help reduce the leakage current of the dielectric film without significantly reducing its capacitance.

This application is a divisional of application Ser. No. 09/296,835,filed on Apr. 22, 1999, which is hereby incorporated by reference.

BACKGROUND

The present invention relates generally to semiconductor devices and,more particularly, to the formation of a dielectric film using a wetrapid thermal oxidation process.

Insulating materials, such as dielectrics, used in semiconductor devicesare selected based on their electrical or other properties and theirintended use. For example, a typical DRAM device can include a firstarea assigned to a memory cell array and a second area assigned toperipheral circuits. The memory cells in the cell array include atransistor coupled in series with a stacked-type or other storagecapacitor. The transistor includes a gate dielectric layer such as anoxide. The storage capacitor, which stores charge to represent data,also includes a dielectric material disposed between two electrodes.

It often is desirable to select a dielectric material having a highdielectric constant for enhanced capacitance. For example, a tantalumoxide (Ta₂O_(x)) film formed by chemical vapor deposition (CVD) has ahigh dielectric constant (ε_(r)) of about 25 to 30. Such a film canprovide good step coverage and can be fabricated relatively easilycompared to other insulating films having high dielectric constants. Thetantalum oxide film typically is deposited as an oxygen-deficient oxide,and such oxygen deficient films typically are leaky. To improve theleakage current properties of the tantalum oxide film, an oxidizingtreatment can be performed following deposition of the film.

Some oxidation processes are performed at elevated temperatures for anextended duration which can result, for example, in the diffusion ofoxidizing species through the dielectric layer so that more oxygen isincorporated into the insulating film. This results in a betterinsulating film. Other defects, however, such as pinholes, can result inleakage current and, therefore, in an insulating film having a breakdownvoltage that is not as high as desirable. Dielectric films with suchdefects do not have sufficiently high capacitance for the memory cellsrequired for DRAMs of 256 megabits and larger. Thus, it is desirable toimprove the techniques for enhancing the properties of tantalum oxideand other dielectric films used in DRAM and other semiconductor devices.

SUMMARY

In general, a method of fabricating a semiconductor device includesdepositing a dielectric film and subjecting the dielectric film to a wetoxidation in a rapid thermal process chamber. The technique can be used,for example, in the formation of various elements in an integratedcircuit, including gate dielectric films as well as capacitive elements.

In one particular aspect, a method of fabricating a semiconductor deviceincludes depositing a dielectric film over an active region of asemiconductor substrate to form a gate of a transistor and subjectingthe dielectric film to a wet oxidation in a rapid thermal processchamber. For example, steam can be provided to a vicinity of thedielectric film while the substrate with the dielectric film is in therapid thermal process chamber.

Similarly, according to another aspect, a method of fabricating acapacitive element includes forming a lower electrode of the capacitiveelement. A dielectric film is deposited over the lower electrode and issubjected to a wet oxidation in a rapid thermal process chamber, forexample, by providing steam to a vicinity of the dielectric film. Anupper electrode then is formed over the dielectric film.

Various implementations include one or more of the following features.The dielectric film initially deposited can be an oxygen-deficient filmand can include a material having a dielectric constant of at leastabout 25. Exemplary dielectric materials that can be deposited includetantalum oxide, silicon nitride, barium strontium titanate, strontiumtitanate, lead zirconium titanate and strontium bismuth tantalate, amongothers. In some implementations, the dielectric film is deposited bychemical vapor deposition.

One of several techniques can be used to provide steam to a vicinity ofthe insulating film. Such techniques include using a bubbled water vaporsystem, a pyrogenic system or a catalytic system, or generating steam inthe chamber in situ.

The temperature, duration and amount of steam can be selected tooptimize the oxidation process to obtain a film that is less prone toleakage. For example, in some implementations, the temperature in therapid thermal process chamber can be about 450° C. or higher. Ingeneral, the wet rapid thermal oxidation can be performed for a durationsuch that the oxidizing species does not diffuse significantly throughthe film. In particular, the duration can be selected so that theoxidizing species does not significantly affect the capacitive and otherproperties of the insulating film and so that it does not oxidize filmsunder the insulating dielectric. Exemplary durations are on the order ofonly several minutes, and typically can be as short as less than oneminute.

In some implementations, it is also desirable to subject the insulatingfilm to a heat treatment in an ambient comprising a stabilizing gas suchas nitrogen, oxygen, nitrogen oxide, or nitrous oxide. The heattreatment can help crystallize or otherwise stabilize the electrical andother properties of the film. The heat treatment can be performed eitherprior to or after subjecting the insulating film to the wet oxidation.In some cases, the heat treatment also is performed in a rapid thermalprocess chamber.

One or more of the following advantages are present in someimplementations. Performing a wet oxidation process in an RTP chamber tocondition the dielectric film can help reduce the leakage current of thefilm without significantly reducing its capacitance. In particular, thetight temperature control provided by the RTP process allows the wetoxidation to be performed quickly so that the oxidizing species does notdiffuse significantly through the dielectric film and reduce oradversely affect the capacitance of the structure.

With respect to the formation of gate dielectric films, using materialswhich have relatively high dielectric constants allows the gatedielectric to be made relatively thick and yet still provide acapacitance whose value is similar to thinner gates. The thickerdielectric layer can help reduce the adverse affects of gate hardeningcaused, for example, by boron penetration. Furthermore, the oxygencontent of the as deposited film can be increased by subjecting the filmto the wet RTP oxidation. The electrical properties of the gatedielectric film can, thus, be enhanced.

Other features and advantages will be readily apparent from thefollowing detailed description, the accompanying drawings, and theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of an exemplary semiconductor device accordingto the invention.

FIGS. 2 and 3 are cross-sectional diagrams illustrating details forfabricating a gate insulating layer according to the invention.

FIG. 4 illustrates an exemplary rapid thermal process (RTP) chamber foruse in the present invention.

FIGS. 5 through 7 are cross-sectional diagrams illustrating details forfabricating capacitive elements of the memory device according to theinvention.

FIG. 8 is a flow chart showing various steps of conditioning aninsulating film according to the invention.

DETAILED DESCRIPTION

Referring to FIG. 1, an exemplary semiconductor memory device 1 includesan n-type well 4 formed in a p-type silicon substrate 2, and a p-typewell 6 formed in the n-type well 4. At the surface of the p-type well 6,a pair of transistors 8 are formed and constitute a memory cell of thememory device 1. Each of the transistors 8 includes n-type source/drainregions 10A, 10B, a gate dielectric film 12, and a gate electrode 14.The gate electrode 14 can include a polycrystalline silicon film 16 anda silicide film 18 stacked above the gate dielectric film 12. Fieldoxide regions 15 separate the transistors 8 from other devices formed onthe semiconductor wafer.

Each transistor 8 is covered with a first interlayer insulating film 20.A contact hole 22 is formed through the insulating film 20 and reachesthe source/drain region 10B common to the pair of transistors 8. A bitline 24 is connected to the source/drain region 10B through the contacthole 22.

The bit line 24 is covered with a second interlayer insulating film 26.Capacitive elements are formed above the insulating film 26. Thestacked-type capacitive elements include a lower electrode 30, acapacitor insulating film 32, and an upper electrode 36. Each of thepaired lower electrodes 30 is electrically connected to a respective oneof the source/drain regions 10A through contact holes 38 which extendthrough the first and second interlayer insulating films 20, 26.

The capacitive elements are covered with a third interlayer insulatingfilm 40, and electrodes 42 are provided on the surface of the thirdinterlayer insulating film.

Further details for fabricating the gate insulating layer 12 aredescribed with reference to FIGS. 2 and 3. Referring to FIG. 2, adielectric film 12A is formed over the surface of the semiconductorwafer, for example, by a CVD process. Preferably, the dielectric filmhas a relatively high dielectric constant ε_(r). For example, a lowpressure CVD apparatus with a source gas comprisingpentaethoxyltanatalum (Ta(OC₂H₅)₅) gas and oxygen can be used to form atantalum oxide (Ta₂O_(x)) film 12A having a dielectric constant ε_(r) ofabout 25 to 30. The deposited film 12A can be either an amorphous orcrystalline film. In other implementations, a silicon nitride(Si_(x)N_(y)) film can be deposited as the film 12A.

After the dielectric film 12A is deposited or grown, the film issubjected to a densifying treatment by which a film 12 (FIG. 3) isformed. The densifying treatment includes heating the semiconductorwafer to stabilize and/or crystallize the film 12A and includessubjecting the wafer to a wet oxidation in a rapid thermal process (RTP)chamber. The wet RTP oxidation process helps raise the oxygen content ofthe resulting film 12. If Si_(x)N_(y) is used as the film 12A, then thewet RTP oxidation treatment will result in a silicon oxynitride film 12.

Using materials such as tantalum oxide or silicon nitride which haverelatively high dielectric constants allows the gate insulating layer tobe made thicker than the typical 30–40 Å and yet still provide acapacitance whose value is similar to the thinner gate layers. Thethicker insulating layer also can help reduce the adverse affects ofgate hardening caused by boron penetration. Furthermore, the oxygencontent of the as-deposited film 12A can be increased by subjecting thefilm 12A to the wet RTP oxidation. The electrical properties of the filmcan, thus, be enhanced.

Referring to FIG. 4, an exemplary RTP chamber 50 for use in theforegoing technique includes a substrate support structure 52. Asemiconductor wafer 54 with the insulating film 12A formed thereon ismounted on the support structure 52 and is heated by a heating element56 located above the wafer. A reflector 58 mounted beneath the wafer 54forms a reflecting cavity for enhancing the emissivity of the wafer.During processing, an inert gas 60 such as argon (Ar) can be provided tothe chamber interior to help purge the chamber 50. Steam 62 for the wetoxidation can be provided from a source external to the chamber 50 orcan be generated in situ within the chamber. Using an RTP process forthe wet oxidation provides precise temperature control and allowsoxidation of the film 12A to be performed quickly. In someimplementations, the RTP chamber 50 in which the wet oxidation iscarried out is part of a cluster system.

The steam for the wet oxidation can be provided to the interior of theRTP chamber in the vicinity of the film 12 in various ways. For example,a bubbled water vapor technique can be used in which a water reservoirexternal to the interior of the RTP chamber is heated to generate steam.The steam then is carried to the interior of the chamber 50 by a carriergas such as Ar.

Alternatively, a pyrogenic system can be used in which hydrogen (H₂) andoxygen (O₂) gases are mixed together and heated by a torch to form steamor water vapor. The vapor then is carried to the chamber interior via aduct.

In yet other embodiments, a catalytic system can be used to generatesteam or water vapor from a mixture of H₂ and O₂ gases. Platinum oranother noble metal can be used as the catalyst.

According to another embodiment, an in situ steam generation process isused in which H₂ and O₂ gases are mixed within the chamber 50. Heat fromthe wafer 50 can serve as the catalyst for forming steam in the vicinityof the wafer.

If a mixture of H₂ and O₂ gases is used to form the steam, suitableratios of H₂ gas to O₂ gas are in the range of about 0.1 to about 0.80.In general, the higher the ratio of H₂ gas to O₂ gas, the moreaggressive is the oxidation process. The ratio of steam relative toother gases in the chamber 50 should be at least as high as 0.005, andpreferably is in the range of about 0.1 to about 0.5, although lesser orgreater amounts also can be used.

In various implementations, temperatures in the range of approximately450° C. to about 1050° C. are suitable for the RTP process. Morespecifically, temperatures greater than about 600° C. are desirable, asare temperatures in the range of about 750° C. to about 950° C.,particularly for crystalline films. For non-crystalline films, however,a lower temperature in the range of about 450° C. to about 750° C. canbe used. The optimal pressure in the chamber will vary depending on theparticular RTP system and wet oxidation technique used. In general, thepressure can be at about atmospheric pressure, although if the H₂ and O₂gases are combined in the chamber 50, then the pressure should be keptlower, for example, around 1 millitorr. The wet oxidation can beperformed for as short a duration as a spike anneal or as long asseveral minutes. Preferably, however, the wet oxidation lasts for aduration in the range of about twenty to sixty seconds.

In some cases it also may be desirable to subject the gate dielectricfilm 12 to a heat treatment in an ambient comprising a stabilizing gas.Such as heat treatment in the presence of a stabilizing gas can beperformed either prior to or subsequent to the wet RTP oxidationprocess. Further details of the heat treatment using a stabilizing gasare discussed below with reference to FIG. 8.

Following formation of the gate dielectric film 12, the gate electrodes14, the first interlayer insulating film 20, the contact hole 22, thebit line 24, the second interlayer insulating film 26, and the contacthole 38 can be formed using known techniques.

Once the second interlayer insulating film 26 and the contact hole 38are formed, a polycrystalline silicon film is deposited, for example,using a CVD process. The polycrystalline film can be doped withphosphorous and patterned to form the lower electrode 30. A rapidthermal nitriding treatment then can be performed to form a siliconSi_(x)N_(y) over the polycrystalline film.

In general, the techniques described above with respect to the gatedielectric film 12 also can be used to form the insulating film 32 forthe capacitive elements. Preferably, the film 32 includes a materialwith a relatively high dielectric constant ∈_(r). Suitable materialsinclude, among others, Ta₂O_(x), Si_(x)N_(y), barium strontium titanate(Sr_(x)Ba_(1-x)TiO₃), strontium titanate (SrTiO₃), lead zirconiumtitanate (PbZrTiO₃) and strontium bismuth tantalate(Sr_(x)Bi_(y)Ta_(z)O₉). For example, as previously noted, Ta₂O_(x) has adielectric constant of about 25 to 30. Sr_(x)Ba_(1-x)TiO₃ can have adielectric constant as high as about 300, although the polar nature ofSr_(x)Ba_(1-x)TiO₃ may make it somewhat less desirable for someapplications.

In one exemplary implementation, a tantalum oxide film 32A (FIG. 5) isformed over the surface of the interlayer insulating film 26 includingthe surface of the lower electrode 30 by a CVD process. As describedabove, a low pressure CVD apparatus with a source gas comprisingpentaethoxyltantalum (Ta(OC₂H₅)₅) gas and oxygen can be used to form thetantalum oxide film 32A. The deposited tantalum oxide film 32A can beeither an amorphous or crystalline film. The thickness of the film 32Acan be in the range of several angstroms (Å) to several hundred Å, andshould be optimized to obtain a desired capacitance.

After the film 32A is deposited, it is subjected to a densifyingtreatment by which the oxide film 32 (FIG. 6) is formed. The densifyingtreatment includes heating the semiconductor wafer to stabilize and/orcrystallize the film 32A and includes subjecting the wafer to a wetoxidation in a rapid thermal process (RTP) chamber. Performing a wetoxidation process in an RTP chamber to form the film 32 can help reducethe leakage current of the dielectric film without significantlyreducing its capacitance. In particular, the tight temperature controlprovided by the RTP process allows the wet oxidation to be performedquickly so that the oxidizing species does not diffuse significantlythrough the dielectric film and distort the capacitance.

The RTP chamber 50 described above can be used to perform the wetoxidation of the film 32A. Furthermore, the various techniques describedabove for providing steam to the RTP chamber interior with respect toformation of the gate dielectric film 12 also can be used to form thecapacitive dielectric film 32. In particular, the steam for the wetoxidation of the film 32A can be provided to the interior of the RTPchamber using a bubbled water vapor technique, a pyrogenic system, or acatalytic system. Alternatively, an in situ steam generation process canbe used. The various process parameters, such as temperature, pressure,and mixtures of gases, discussed above with respect to formation of thegate dielectric 12 can be used during wet oxidation of the film 32A aswell.

In some applications, the RTP process performed for the wet oxidation issufficient to stabilize the gate dielectric film 12 or the film 32 asdesired. For example, if an amorphous film 12 or 32 is desired, the wetRTP oxidation may suffice. However, in some cases, it is desirable tosubject one or both of the films 12, 32 to a heat treatment in anambient having a stabilizing gas comprising nitrogen (N₂), oxygen (O₂ orO₃), nitrogen oxide (NO), or nitrous oxide (N₂O) just prior to or afterperforming the respective RTP wet oxidation (see FIG. 8). Such astabilizing process can be used, for example, to obtain a crystallinefilm. Although the stabilizing process can be performed in a separatefurnace, performing the stabilizing process in the same or a differentRTP chamber can provide tighter temperature control. The total timerequired for conditioning the dielectric film 12 (or 32) using the RTPprocess can be on the order of several minutes per wafer.

In some implementations, the stabilizing process can is performed whilethe temperature in the RTP chamber is brought to the temperature for thewet oxidation. The wet oxidation should be performed at a temperatureslightly less than the temperature for the stabilizing process so thatthe properties of the film 12 (or 32) obtained during the stabilizingprocess are not adversely affected by the subsequent RTP wet oxidation.Thus, in one implementation, the dielectric film 32A initially issubjected to an ambient comprising N₂ at a temperature greater thanabout 750° C. The wet RTP oxidation subsequently is performed at atemperature in the range of about 500 to 700° C. In an alternativeembodiment, the film 32A initially is subjected to the wet RTP oxidationat a temperature in the range of about 500 to 700° C. Subsequently, theoxidized film 32 is subjected to dry N₂ or O₂ at a temperature greaterthan about 700° C. As previously mentioned, the wet oxidation combinedwith the higher temperature stabilization can be used to form acrystalline dielectric film.

Once the densification process for the film 32 is complete, the upperelectrode 36 is formed, for example, by depositing and patterning atitanium nitride film as shown in FIG. 7.

Although the techniques have been described with respect to memory cellssuch as DRAMs, the techniques also can be used with other semiconductordevices incorporating insulating or dielectric films, particularly wherethe as-deposited insulating film is oxygen deficient or more leaky thandesired. The foregoing techniques can be used to enhance the oxygencontent of the as-deposited dielectric film without adversely affectingother electrical properties of the film or the interface between thedielectric film and an underlying layer.

Other implementations are within the scope of the following claims.

1. A method of fabricating a semiconductor device, the methodcomprising: depositing a dielectric film over an active region of asemiconductor substrate to form part of a gate of a transistor;subjecting the dielectric film to a densifying treatment to stabilizesaid film by heating the semiconductor substrate in a rapid thermalchamber at a first temperature; and subjecting said stabilizeddielectric film to a wet oxidation with steam process in the rapidthermal process chamber at a second temperature to raise the oxygencontent of said stabilized dielectric film, said steam being carried tothe rapid thermal process chamber, wherein the first and secondtemperature of the rapid thermal process chamber is from approximately450° C. to about 1050° C., wherein said stabilized dielectric film issubjected to said wet oxidation with steam process for a duration ofabout 20 seconds to about 60 seconds, wherein the ratio of steam toother gases in the rapid thermal process chamber is in the range fromabout 0.1 to about 0.5 and the pressure of said rapid thermal processchamber is held at about atmospheric pressure, and wherein said firsttemperature is greater than said second temperature so that propertiesof the stabilized dielectric film are not adversely affected by the wetoxidation with steam process.
 2. The method of claim 1 wherein the wetoxidation with steam process is performed at a temperature in the rangeof about 750° C. to about 950° C.
 3. The method of claim 1 whereindepositing a dielectric film includes depositing a material having adielectric constant of at least about
 25. 4. The method of claim 1wherein depositing a dielectric film includes depositing a materialselected from the group consisting of tantalum oxide and siliconnitride.
 5. A method of fabricating a semiconductor device, the methodcomprising: depositing a dielectric film over a semiconductor substrateto form one of a gate and a capacitor dielectric; subjecting thedielectric film to a densifying treatment to stabilize said film byheating the semiconductor substrate at a temperature greater than about700° C.; and subjecting the dielectric film to a wet oxidation withsteam process to raise the oxygen content of said dielectric filmprovided by heating a mixture of hydrogen and oxygen gases in a rapidthermal process chamber at a temperature greater than about 450° C.,wherein said mixture is a ratio from 0.1 to approximately 0.80 ofhydrogen gas to oxygen gas and combined in said rapid thermal processchamber, and said rapid thermal process chamber has a pressure of around1 millitorr, wherein the temperature for the wet oxidation with steamprocess is lower than the temperature for the densifying treatment sothat properties of the stabilized dielectric film are not adverselyaffected by the wet oxidation with steam process.